Process for the production of a substitute natural gas

ABSTRACT

Substitute Natural Gas is produced by passing methanol vapour, optioally admixed with a minor proportion of recycle carbon dioxide, through a bed of nickel catalyst in an isothermal reactor and removing carbon dioxide from the product gas, the methanol vapour being passed into the catalyst bed at a temperature of at least 250 DEG C, preferably about 250 DEG C, and the bed being maintained at a temperature of from 250 DEG C to 350 DEG C, preferably about 300 DEG C, by external cooling with boiling water at a steam pressure of at least 550 psig.

This invention relates to a process for the production of a substitutenatural gas (SNG) and in particular to such a process using methanolfeedstock.

Methanol as an SNG feedstock has the following in-built advantages:

1. It is clean, free of sulphur, and any gum forming or coke formingsubstances;

2. It is therefore readily vapourized and requires no desulphurizationstep before being passed to a nickel catalyst;

3. It is a partially oxygenated compound and may be regarded as CH₂.H₂O. It thus carries an in-built water supply as far as any steamreforming process is concerned;

4. If one considers the ideal overall stoichiometry of the conversion ofmethanol to methane viz.

    CH.sub.3 OH = 0.75CH.sub.4 + 0.25CO.sub.2 + 0.5H.sub.2 O

it is readily appreciated that the process is a net generator of highquality water, thus no process water supply is required. Under oxygenfree conditions, no deaeration or scavenging step is needed;

5. Methanol is capable of drying gases and of carbon dioxide removal.

In addition to the direct advantages of methanol as an SNG feedstock theproperties of methanol are such that it constitutes an economical andconvenient energy transfer medium in that it may be readily transportedby pipeline without many of the practical difficulties and high costinvolved in gas pipelines or pipelines for other liquids such as heavyoils. Other materials such as coal and light and heavy oils have beenproposed as SNG feedstocks but the use of such materials is in manyinstances complicated by the necessity to transport the feedstock to thearea of gas demand or indeed to manufacture the gas at source, forexample, at the minehead, and then to transport the gas by pipeline tothe area of gas demand. Thus it may be advantageous to convert suchmaterials or other inconvenient solid, liquid or gaseous energy sources,often by well proven technology, to methanol for transportation, eitherby pipeline or sea tanker, and thereafter to manufacture SNG directlyfrom the methanol in the area of gas demand. Previous processes for theproduction of SNG using methanol feedstock have involved the followingsteps:

A. initial gasification of methanol over a nickel catalyst to an outlettemperature of 500°-550°C;

b. cooling of gases so produced which contain mainly methane, hydrogenand carbon dioxide with less than 1% carbon monoxide, in the presence ofsteam, to 250°-300°C. A typical dry gas analysis would be:

                   % Vol.                                                         ______________________________________                                        CH.sub.4         65.0                                                         CO.sub.2         21.5                                                          H.sub.2         13.0                                                         CO               0.5                                                                           100.0                                                        ______________________________________                                    

c. adiabatic methanation of said gases to give a gas containing about 2per cent water;

d. further cooling to 250°-300°C and final methanation to give a gas ofa 1000 BTU/FT3 after carbon dioxide removal;

e. carbon dioxide removal by hot carbonate washing or other means;

f. gas drying, e.g. by a glycol wash process.

Methanol gives a high temperature rise on the initial gasification step(step (a)) and this necessitates the use of a high steam ratio in orderto avoid carbon deposition in the outlet gas and further to avoidunacceptably high outlet temperatures from the reactor giving rise toproblems in reactor design. This route may be improved by incorporatinga number of adiabatic reactors in series with methanol addition andsteam raising between each stage. Low steam ratios may then be achievedwithout excessive outlet temperatures.

We have now found that the production of SNG from methanol may becarried out at improved thermal efficiency and with less complicatedplant design by using an isothermal reactor as opposed to an adiabaticreactor.

Accordingly, the present invention provides a process for the productionof SNG from methanol feedstock which process comprises passing methanolvapour optionally admixed with a minor proportion of recycle carbondioxide through a bed of nickel catalyst in an isothermal reactor andremoving carbon dioxide from the product gas, the methanol vapour beingpassed into the catalyst bed at a temperature of at least 250°Cpreferably about 250°C and the bed being maintained by the reaction at atemperature of from 250°C to 350°C preferably about 300°C.

Experiment has shown that the reaction of methanol vapour to form SNGtakes place readily over a nickel catalyst at 250°C. Thus, provided thereaction temperature remains at all times about 250°C, there is notendancy for the reaction to `quench`. The overall reaction is in factstrongly exothermic providing heat in excess of that required tomaintain the catalyst bed in the required temperature range of from 250°to 350°C. The surplus heat or as much of the surplus heat as isnecessary is preferably utilized in the process of the present inventionfor vapourizing and preheating further methanol feedstock to therequired temperature of at least 250°C prior to entry into the reactor.

One form of isothermal reactor which may be conveniently employed in theprocess of the present invention comprises a plurality of tubessurrounded by a shell such that when the reactor is in use catalyst maybe contained in the tubes and boiling water may be circulated throughthe shell at an appropriate rate thereby conducting away surplus heatgenerated in the reaction tubes and maintaining isothermal or nearisothermal reaction conditions.

An alternative and preferred form of isothermal reactor for use in theprocess of the present invention comprises an inner tube sheathed by anouter tube concentric with said tube such that when the reactor is inuse catalyst may be contained in the inner tube and boiling water may becirculated through the annular space between the inner and outer tubesto maintain isothermal or near isothermal conditions for the reactionoccurring in the inner tube. This arrangement enables high heat transferareas to be obtained without the need for large pressure vessels.However the tubes of the concentric tube reactor are limited in size bypractical considerations, for example to an inner tube having an insidediameter of about 4 inches, thus in a preferred embodiment, to secureadequate output, a plurality of concentric tube reactors may beemployed. In such an arrangement all the reactors, optionally containedin a mild steel casing and insulated from one another for example withpurlite insulation, may be piped separately to inlet and outlet headersfor both feedstock and cooling water.

At steam pressures above 550 psig. the boiling point of water is above250°C. Thus by raising steam in the above described isothermal reactorsat 550 psig. or higher pressure, reaction quenching may be prevented andthe temperature rise of the reactants may at the same time be limitedsuch that the temperature of the catalyst bed and the outlet temperatureof products is close to 250°C and lies in the required range of from250° to 350°C. At temperatures in this range methanol is completelyconverted to methane and carbon dioxide with less than 1% hydrogen indry gas product and minor traces of carbon monoxide (˜0.1%) only.

Circulation of boiling water through the reactors may be effected bymeans of a pump or by natural circulation. As stated above, some of thesurplus heat conducted away by the boiling water is preferably utilizedin the process of the present invention to vapourize and preheatmethanol feedstock, according to known techniques. The remainder of thesurplus heat may be used, for example, to drive turbines or othermachinery.

As catalyst in the process of the present invention may be employed, forexample, any of the nickel catalysts conventionally used in thegasification of methanol. A preferred catalyst in Catalytic Rich Gas(CRG) catalyst as described in British Patent Specification No. 820,257and which consists of reduced nickel activated with alumina, andcontains, for example, 15% of nickel. Such a catalyst may be prepared bytreating an aqueous solution of water-soluble salts, for example, thenitrates, of nickel and aluminium with an alkali, such as sodiumcarbonate, to produce a precipitate of a mixture of nickel and aluminiumcompounds, washing and drying the precipitate, reducing the nickelcompound to metallic nickel, and granulating or pelleting the resultingmixture of reduced nickel and alumina. Alternatively, granules of activealumina may be impregnated with an aqueous solution of nickel nitrate,then roasted, and the nickel oxide reduced to metallic nickel.

Removal of carbon dioxide from the gas produced in the reactor, usuallyafter cooling of the gas, may be effected by any known technique. Forinstance the gas may be partially scrubbed of carbon dioxide using a`coarse` process such as water washing or `flash` Benfield. Cryogenicseparation may be used to bring the level of CO₂ down to 6 to 10%volume. The remainder of the carbon dioxide may be removed usingmethanol feedstock scrubbing which depends in principle on the highsolubility of carbon dioxide in methanol at high pressures and lowtemperatures. Recycle carbon dioxide contained in methanol feedstockwhich has been obtained in this way has little effect on the equilibriumobtained at the outlet of the reactor.

After removal of carbon dioxide, preferably followed by enrichment inconventional manner, the gas obtained by the process of the presentinvention is intended to be interchangeable with natural gas.

It is to be understood that the invention also includes SNG whenproduced by the process of the invention and apparatus for producing SNGas specifically described herein.

The invention will now be further illustrated by the following examplesof which Example 1 is a comparative example relating to the knownadiabatic process and Example 2 relates to the isothermal process of thepresent invention.

EXAMPLE 1 Multistage Adiabatic Process

A typical flow diagram for this process is shown in the accompanyingFIG. 1.

The object of this process is to limit the temperature rise in eachreactor to 250°C by interstage cooling. Initially water and methanol aremixed in the ratio 4 lbs./lb vapourised and fed to the first adiabaticreactor at a preheat of 250°C. Gases leave the reactor in equilibrium at500°C and are cooled to 250°C. Cooling may be achieved wholly by a steamraising waste heat boiler or partly by a boiler and partly by directinjection of methanol feed. In this example liquid methanol at ambienttemperature is used and this, of course, eliminates the need for amethanol vapouriser. The amount of methanol added is arranged so as togive a 250°C temperature rise in the next adiabatic reactor. Temperaturecontrol is achieved by bypass control of the waste heat boiler. Theprocess is continued in a similar manner for any number of reactors thenumber being limited by complexity of plant and reactor sizing. Theoutlet temperature of 500°C from the final reactor is not compatiblewith the production of a satisfactory substitute natural gas and afurther two adiabatic stages are required. In the first stage the gas iscooled to 250°C but no further methanol is added. The resultant outlettemperature is in the range 300°-350°C depending on the number ofpreceding stages. This gas is then cooled to a temperature at whichwater is condensed and all the water that was originally added to themethanol is rejected. The gas is reheated to 250°C and passed to a finalreactor where equilibrium is reached at an outlet temperature in therange 260°-300°C. The final gas, after carbon dioxide removal to 1% andenrichment to 1000 Btu/scf is a fully interchangeable substitute naturalgas. CRG catalyst would be used throughout the process. The majoradvantage of the process is the high thermal efficiency of using themultiple reactor arrangement in that the greater the stages employed theless water is required to be vapourised per unit mass of methanolgasified. If more than four stages are used the process is selfsupporting in heat requirement and surplus steam may be expected or usedfor power generation.

The composition of the gas produced at each stage of the processillustrated in FIG. 1 is given in the following Table 1.

                                      TABLE 1                                     __________________________________________________________________________    Ex Stage 1  Ex Stage 2                                                                            Ex Stage 3                                                                            Ex Stage 4                                                                            Ex Stage 5                                Wet     Dry Wet Dry Wet Dry Wet Dry Wet Dry                                   __________________________________________________________________________    CO.sub.2                                                                          4.4 24.4                                                                              6.3 24.6                                                                              7.9 24.7                                                                              9.3 24.3                                                                              10.6                                                                              24.0                                  CO  0.1 0.6 0.1 0.4 0.2 0.6 0.2 0.5 0.2 0.5                                   H.sub.2                                                                           6.8 37.8                                                                              6.8 26.6                                                                              6.8 21.2                                                                              6.5 17.0                                                                              6.3 14.3                                  CH.sub.4                                                                          6.7 37.2                                                                              12.4                                                                              28.4                                                                              17.1                                                                              53.5                                                                              22.3                                                                              58.2                                                                              27.1                                                                              61.2                                  H.sub.2 O                                                                         82.0                                                                              --  74.4                                                                              --  68.0                                                                              --  61.7                                                                              --  55.8                                                                              --                                    C.sub.3 H.sub.8                                                                   --  --  --  --  --  --  --  --  --  --                                    __________________________________________________________________________                Ex Stage 6                                                                            Ex Stage 7                                                                            Final gas ex Stage 8                                                                    After                                                                           After                                             Wet Dry Wet Dry Wet Dry   CO.sub.2                                                                        enrich-                                                                     re-                                                                           moval                                                                           ment                                  __________________________________________________________________________            CO.sub.2                                                                          11.5                                                                              24.6                                                                              10.6                                                                              25.0                                                                              16.7                                                                              25.0  1.0                                                                             1.0                                           CO  0.3 0.6 0.0 0.0 0.0 0.0   0.0                                                                             0.0                                           H.sub.2                                                                           6.2 13.3                                                                              0.7 1.7 0.2 0.3   0.4                                                                             0.4                                           CH.sub.4                                                                          28.7                                                                              61.5                                                                              31.1                                                                              73.3                                                                              49.9                                                                              74.7  98.6                                                                            97.2                                          H.sub.2 O                                                                         53.3                                                                              --  57.6                                                                              --  33.2                                                                              --    --                                                                              --                                            C.sub.3 H.sub.8                                                                   --  --  --  --  --  --    --                                                                              1.4                                   __________________________________________________________________________

EXAMPLE 2 Single Stage `Isothermal` Process

In this process the temperature rise due to the exothermic reaction iscontrolled by directly cooling the catalyst vessel with a jacket ofboiling water and steam. This technique enables methanol to be gasifieddirectly without the necessity to add water to the feed. This example isalso used to demonstrate the use of methanol to recycle carbon dioxideround the process. Methanol containing 20 mole percent of carbon dioxideis vapourised and preheated to 250°C prior to introduction to theisothermal reactor. The reactor (the design of which is discussedhereinbelow) is cooled with boiling water at 850 lb/in.² and the flowsare adjusted to give an outlet temperature of 300°C. The gases arecooled and passed to the first stage of carbon dioxide removal. There isinsufficient heat in the gases to vapourise and preheat the feed andthis must therefore be accomplished by heat exchange with steam. In thecoarse carbon dioxide removal unit, carbon dioxide formed from themethanol feed is removed. The recycle carbon dioxide is then removed inthe `Rectisol` unit leaving a gas which is substantially free of carbondioxide and which after enrichment to 1000 Btu./scf, is interchangeablewith natural gas.

Two types of reactor design may be used:

a. the catalyst may be contained in tubes in a bundle contained in apressure shell through which the cooling water and steam circulates. Themajor limitation with this design is the size of the outer shell. Forexample a 10 mmscfd plant would have a heat release of 64.10⁶ Btu./hr.in the reactor. For an assumed heat flux of 1000 Btu./ft.² a heattransfer area of 6400 ft.² is required. If 3 inch I.D., 3.5 inch O.D.tubes 20 ft. long are used, 407 tubes would be required and the methanolloading to the catalyst would be 2250 lb/ft.² hr. If the tubes arearranged on a 5 inch triangular pitch the shell would be 9 ft. I.D. and9 ft. 6 in. O.D. This type of design results in comparatively low heattransfer area to cross sectional area ratio because of practical andeconomic restrictions on the outer pressure shell. This ratio cannot beimproved by reducing the catalyst tube diameter beyond 3 inches becauseof difficulties with loading and unloading of catalyst. The length ofthe reactor could be extended but such a vessel 100 ft. long for examplewould be very expensive.

b. The catalyst may be contained in tubes each of which is surrounded byanother tube with boiling water in the annular space between them. Thiseliminates the problems with a large pressure vessel and the loading tothe catalyst, the length of the tubes and the number of tubes can all beincreased. For example a 50 mmscfd plant with a loading of 7000 lb/ft.²hr to the tubes and an assumed heat flux of 1000 Btu./ft² hr would have346 4.5 inch O.D. 4 inch I.D. tubes 100 ft. long each surrounded by acooling water tube. The catalyst tubes would probably contain 1/4 inchCRG catalyst. Such an arrangement could be contained in a mild steelcasing with purlite insulation between the tubes and the casing sizewould be approximately 12 ft. by 20 ft. Each tube would be pipedseparately to inlet and outlet headers for both gas and cooling waterand the water would be circulated by pumps to a steam drum situatedabove the tubes.

A typical flow diagram for the process of the invention incorporating areactor as described at (b) above is shown in the accompanying FIG. 2.The composition of the gas produced at various stages of this process ispresented in the following Table 2.

                                      TABLE 2                                     __________________________________________________________________________    Ex Cooled Catalyst                                                                            Ex Primary                                                                            Ex Secondary                                                                          Product                                       Tubes           CO.sub.2 Removal                                                                      CO.sub.2 Removal                                                                      Gas                                           __________________________________________________________________________    Wet       Dry   Dry     Dry     After                                                                 Enrichment                                            __________________________________________________________________________    CO.sub.2                                                                          26.50 37.43 21.57   1.00    0.98                                          CO  0.01  0.01  0.01    0.02    0.02                                          H.sub.2                                                                           0.35  0.49  0.61    0.78    0.77                                          CH.sub.4                                                                          43.95 62.07 77.81   98.20   96.66                                         H.sub.2 O                                                                         29.19 --    --      --      --                                            C.sub.3 H.sub.8                                                                   --    --    --      --      1.57                                          __________________________________________________________________________

It is believed that in addition to methanol gasification to SNG, otherexothermic reactions could be profitably carried out in the apparatus ofthe present invention employing an isothermal reactor. Examples of suchreactions would be methanation of carbon oxides to convert a lean gas,such as LURGI gas, to SNG and reduction of hydrogen content of GRHeffluent or FBH effluent. The invention could also be applied to thedirect gasification of naphtha feedstocks provided the reactor feed iscomprised, in part, of elemental hydrogen. The presence of hydrogenreduces the "quench" temperature of the gasification reaction so that,although the reactants may be cooled to the jacket temperature bypassage over a layer of deactivated catalyst, the hydrogen enables thereaction to proceed. The source of hydrogen for this type of processwould be typically the effluent from a CRG reactor or the effluent froma high temperature tubular reformer.

I claim:
 1. A process for the production of gases suitable forsubstitution for Natural Gas which process comprises preheating methanolto a temperature of at least 250°C to form a methanol vapor, passingsaid vapor through a bed of nickel catalyst in an isothermal reactor toform a product gas, maintaining said catalyst bed at a single isothermaltemperature within the range of 250° to 350°C by external cooling withboiling water at a steam pressure of at least 550 psig, and removingcarbon dioxide from said product gas.
 2. A process as claimed in claim 1wherein the carbon dioxide removed from the reactor product gases isrecycled to the reactor inlet.
 3. A process as claimed in claim 1wherein carbon dioxide is removed from the reactor product gases to alevel of 6 to 10% volume by cryogenic separation and the remainder isremoved by scrubbing with methanol feedstock.